CVE-2021-21704 Overview
CVE-2021-21704 is a memory corruption vulnerability affecting the PHP Firebird PDO driver extension. When PHP applications connect to a malicious Firebird database server, the server can return invalid response data that is not properly parsed by the driver. This improper handling can trigger crashes in various database functions including getAttribute(), execute(), fetch(), and others, resulting in denial of service or potentially memory corruption.
The vulnerability stems from insufficient validation of database server responses, classified under CWE-125 (Out-of-Bounds Read) and CWE-787 (Out-of-Bounds Write), indicating that the driver may read or write data outside allocated memory boundaries when processing malformed server responses.
Critical Impact
A malicious Firebird database server can cause application crashes, denial of service, or potentially exploit memory corruption to compromise PHP applications using the vulnerable PDO driver.
Affected Products
- PHP versions 7.3.x below 7.3.29
- PHP versions 7.4.x below 7.4.21
- PHP versions 8.0.x below 8.0.8
- NetApp Clustered Data ONTAP
Discovery Timeline
- 2021-10-04 - CVE-2021-21704 published to NVD
- 2024-11-21 - Last updated in NVD database
Technical Details for CVE-2021-21704
Vulnerability Analysis
The vulnerability exists in the PHP Firebird PDO driver's response parsing logic. When processing data returned from a Firebird database server, the driver fails to adequately validate the integrity and format of the response before attempting to parse it. This lack of proper input validation creates conditions where malformed data can cause out-of-bounds memory access.
The issue affects multiple database interaction functions within the PDO Firebird extension. Functions such as getAttribute(), execute(), and fetch() all rely on the underlying response parsing mechanism, making them all potential vectors for triggering the vulnerability. When exploited, these conditions can lead to crashes (denial of service) or potentially memory corruption that could be leveraged for further attacks.
Root Cause
The root cause is improper input validation when parsing database server responses in the PHP Firebird PDO driver. The driver does not sufficiently verify that response data conforms to expected formats and sizes before processing, allowing out-of-bounds read (CWE-125) and out-of-bounds write (CWE-787) conditions to occur. Multiple related issues were identified and tracked in PHP bug reports #76448, #76449, #76450, and #76452.
Attack Vector
The attack requires a network-accessible malicious Firebird database server that a vulnerable PHP application connects to. The attacker must control or compromise a database server that the target application trusts and communicates with. When the PHP application executes database operations, the malicious server responds with crafted invalid data designed to trigger the parsing vulnerabilities.
While the attack complexity is high due to the requirement of controlling a database server that the target application connects to, no privileges or user interaction are required once the malicious server is in position. The vulnerability does not affect confidentiality or integrity directly but has a high impact on availability through denial of service.
The attack scenario involves configuring a malicious Firebird database server to return specially crafted responses that exploit the parsing weaknesses. When the victim PHP application connects and performs database operations like executing queries or fetching results, the malformed responses trigger out-of-bounds memory access, causing the application to crash.
Detection Methods for CVE-2021-21704
Indicators of Compromise
- Unexpected crashes or segmentation faults in PHP processes using PDO Firebird connections
- PHP error logs showing memory access violations during database operations
- Abnormal process terminations when executing getAttribute(), execute(), or fetch() PDO functions
- Core dumps indicating memory corruption in PDO Firebird driver code paths
Detection Strategies
- Monitor PHP application logs for recurring crashes or unexpected terminations related to Firebird database operations
- Implement application-level monitoring to detect repeated PDO connection failures or function call exceptions
- Use process monitoring to identify PHP worker processes experiencing abnormal termination patterns
- Deploy network monitoring to detect suspicious or malformed Firebird protocol traffic
Monitoring Recommendations
- Enable detailed PHP error logging for PDO operations to capture crash diagnostics
- Configure application performance monitoring to alert on elevated error rates in database-related code
- Implement health checks that verify Firebird database connectivity and response integrity
- Monitor system logs for segmentation faults or memory-related errors in PHP processes
How to Mitigate CVE-2021-21704
Immediate Actions Required
- Upgrade PHP to version 7.3.29 or later for the 7.3.x branch
- Upgrade PHP to version 7.4.21 or later for the 7.4.x branch
- Upgrade PHP to version 8.0.8 or later for the 8.0.x branch
- Review and validate the trustworthiness of all Firebird database servers that PHP applications connect to
Patch Information
Security patches addressing this vulnerability were released by PHP in versions 7.3.29, 7.4.21, and 8.0.8. The patches improve validation of database server responses in the Firebird PDO driver to prevent out-of-bounds memory access.
For detailed information about the underlying issues, refer to the PHP bug reports:
Additional vendor advisories are available from Gentoo GLSA 202209-20 and NetApp Security Advisory ntap-20211029-0006.
Workarounds
- Disable the PDO Firebird extension if not required by removing or commenting out the extension directive in php.ini
- Implement network-level controls to restrict PHP application connections to known trusted Firebird database servers only
- Consider using alternative database drivers or database systems if Firebird is not strictly required
- Deploy PHP applications in isolated environments to limit the impact of potential crashes or exploitation
# Disable PDO Firebird extension in php.ini
# Comment out or remove the following line:
# extension=pdo_firebird
# Verify the extension is disabled
php -m | grep -i firebird
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
